A video picture coded in the High Efficiency Video Coding (HEVC) standard may be partitioned in one or multiple slices, and each slice is partitioned into multiple coding tree units (CTU). In the main profile of HEVC, minimum and the maximum sizes of CTUs are specified by syntax elements in the sequence parameter set (SPS). Each CTU consists of a luma coding tree block (CTB) and corresponding chroma CTBs and syntax elements. The size of a luma CTB is chosen from 16×16, 32×32, or 64×64, with the larger sizes typically enabling better compression rate for simple or smooth texture areas. CTUs within a slice are processed according to a raster scan order.
HEVC supports partitioning of a CTU into multiple coding units (CUs) to adapt to various local characteristics using a quadtree partition process. For a 2N×2N CTU, it can be a single CU or can be split into four smaller blocks of equal sizes (i.e., N×N). The quadtree partitioning process recursively splits each CTU into smaller blocks until reaching leaf nodes of the quadtree coding tree. The leaf nodes of the quadtree coding tree are called the CUs in HEVC. The decision whether to code a picture area using Inter-picture (temporal) prediction or Intra-picture (spatial) prediction is made at the CU level. Since the minimum CU size can be 8×8, the minimum granularity for switching between different basic prediction types is 8×8.
One, two, or four prediction units (PU) are specified for each CU according to one of the PU splitting types as shown in FIG. 1, where PU works as a basic representative block for sharing prediction information. FIG. 1 illustrates eight different PU splitting types supported in HEVC including symmetrical and asymmetrical splitting types. Inside each PU, the same prediction process is applied and relevant prediction information is transmitted to the decoder on a PU basis. After obtaining prediction residues by applying the prediction process on the PUs, a CU is partitioned into transform units (TUs) according to another quadtree structure, which is analogous to the quadtree coding tree used for obtaining the CUs from the LCU. The TU is a basic representative block of residues or transform coefficients for applying transform and quantization. The transformed and quantized residual signal of the TUs is encoded and transmitted to the decoder after transform and quantization on a TU basis.
Similar to the definition of CTB, coding block (CB), prediction block (PB), and transform block (TB) are defined to specify the sample array of a luma component or a chroma component associated with CU, PU, and TU, respectively. The quadtree partitioning process is generally applied simultaneously to both luma and chroma components, although exceptions apply when certain minimum sizes are reached for chroma components.